In general, stainless steels are more difficult to machine than carbon and low-alloy steels because stainless steels have high strength and work-hardening rates compared to the carbon and low alloy steels. Consequently, it is necessary to use higher powered machines and lower machining speeds for machining the known stainless steels than for machining carbon and low-alloy steels. In addition, the useful life of a machining tool is often shortened when working with the known stainless steels.
In order to overcome the difficulties in machining the known stainless steels, some grades of stainless steels have been modified by the addition of elements such as sulfur, selenium, phosphorus, or lead. For example, AISI Type 303 stainless steel is a resulfurized, austenitic stainless steel having the following composition in weight percent:
______________________________________ wt. % ______________________________________ C 0.15 max Mn 2.00 max Si 1.00 max P 0.20 max S 0.15 min Cr 17.0-19.0 Ni 8.0-10.0 Fe Balance ______________________________________
Type 303 stainless steel is known to be useful for applications which require good machinability and nonmagnetic behavior, in combination with good corrosion resistance. However, a need has arisen for an austenitic stainless steel having significantly better machinabiiity than Type 303 stainless steel, particularly under production-type machining operations such as on an automatic screw machine.
U.S. Pat. No. 4,784,828 (Eckenrod et al.) relates to a resulfurized Cr-Ni austenitic stainless steel in which the total amount of carbon plus nitrogen is restricted to 0.065 w/o max. The data presented in the patent appears to show that the alloy provides improved machinability in short term laboratory tests because of the restricted amount of carbon and nitrogen. However, it has been discovered that the alloy disclosed in the '828 patent has less than desirable machinability under production-type machining conditions such as are encountered on an automatic screw machine. Furthermore, an austenitic stainless steel in which the carbon and nitrogen are reduced as taught in the '828 patent, provides an undesirably high magnetic permeability, in the cold drawn condition.
Given the foregoing, it would be highly desirable to have an austenitic stainless steel that provides a better combination of magnetic permeability and machinability than is provided by the known austenitic stainless steels.